The columns of the information schema views use special data
types that are defined in the information schema. These are
defined as simple domains over ordinary built-in types. You
should not use these types for work outside the information
schema, but your applications must be prepared for them if they
select from the information schema.

These types are:

cardinal_number

A nonnegative integer.

character_data

A character string (without specific maximum
length).

sql_identifier

A character string. This type is used for SQL
identifiers, the type character_data
is used for any other kind of text data.

time_stamp

A domain over the type timestamp with
time zone

Every column in the information schema has one of these four
types.

Boolean (true/false) data is represented in the information
schema by a column of type character_data
that contains either YES or NO. (The information schema was invented before
the type boolean was added to the SQL
standard, so this convention is necessary to keep the information
schema backward compatible.)